Tried this on both Mac OS X (10.8) with chrome and Windows 7 with firefox.

Its matters more what os your server is as file type detection is done on the server side. (Sometimes mw uses the underlying os for file type detection. Im not sure what the default is off the top of my head)

Mw hasnt overly been tested with file formats other than standard image formats so its probably just an oversight.

(As quick hack work arround - if its important to make this work quickly you could change csv to be associated with text/plain in includes/mime.types . Obviously that is not a proper fix)